1. Field of the Invention
The present invention relates to an electro-optic device that has a driving circuit which may consist of a memory circuit and a pixel driver that is provided for each pixel and that controls pixel display according to a data signal held in the memory circuit, and to electronic equipment, such as office automation equipment and portable equipment in which the electro-optic device is installed.
2. Description of Related Art
In recent years, as an information display device of portable equipments or the like, including a portable telephone and a portable information terminal, a liquid crystal device, which is an example of an electro-optic device, has been in use. The contents of displayed information have been conventionally displayed in characters. These days, however, dot-matrix liquid crystal panels have been used to display more information at a time, and the number of pixels is gradually increasing with a consequent higher duty.
Hitherto, for the above portable equipment, a passive matrix liquid crystal device has been used as a display device. However, a passive matrix liquid crystal device requires a higher voltage with an increasing duty for a selection signal of a scanning line when performing multiplex drive, posing a serious problem in battery-driven portable equipment that is strongly required to minimize power consumption.
To solve such a problem, there has been proposed a static drive liquid crystal device in which one of a pair of substrates constituting a liquid crystal panel is formed of a semiconductor substrate, and a memory circuit shown in FIG. 12 is formed on the semiconductor substrate for each pixel to conduct display control based on data held in the memory circuit. In conjunction with FIG. 12, an operation of a conventional static drive liquid crystal device will now be described.
A scanning line drive circuit 410 is controlled by a scanning line drive circuit control signal 418, and a selection signal (scanning signal) is output to a selected scanning line 409-n (xe2x80x9cnxe2x80x9d is a natural number denoting a number of scanning lines). Likewise, a data line drive circuit 413 is controlled by a data line drive circuit control signal 419, and data signals are supplied to a selected pair of data lines 411-m and 412-m (xe2x80x9cmxe2x80x9d is a natural number denoting a number of data lines) so that they have mutually opposite phases (complementary signals).
At an intersection of the scanning line 409-n and the pair of data lines 411-m and 412-m, a circuit connected to those lines constitutes a pixel. n-channel MOS switching circuits 401 and 402 connected to the scanning line 409-n and the pair of data lines 411-m and 412-m are set to a conducting state when the scanning line 409-n is selected and a selection signal is supplied, and write complementary data signals of the pair of data lines 411-n and 412-m to a memory circuit 403. The memory circuit 403 has two inverters in feedback connection. Then, the scanning line 409-n is set at a nonselective potential and the pair of data lines 411-m and 412-m are set at a high impedance to thereby place the switching circuits 401 and 402 in a nonconducting state, and the data signals written to the memory circuit 403 are retained.
A liquid crystal pixel driver 404 composed of two transmission gate circuits is controlled by potential levels of a first node in the memory circuit 403 and a second node at an inverted level of a potential level at a point of connection of the first node. A first transmission gate circuit is connected to a first voltage signal line 416 and conducts according to a level of a data signal held by the memory circuit 403, and applies a first voltage 414 to a pixel electrode 406. On the other hand, a second transmission gate circuit is connected to a second voltage signal line 417 and conducts according to a level of a data signal held by the memory circuit 403, and applies a second voltage 415 to the pixel electrode 406. To be more specific, if the held data signal is at an H-level, then the first voltage signal line 416 that sets a liquid crystal layer 407 of a liquid crystal pixel driver 404 to an ON state in the case of a normally white display mode conducts, causing the first voltage 414 to be supplied to the pixel electrode 406 via the first transmission gate circuit of the liquid crystal driver 404, so that the liquid crystal pixel 405 is set to in a black display mode by a potential difference from a reference voltage 420 supplied to a common electrode 408. Similarly, if the held data signal is at an L-level, then the second voltage signal line 417 that sets the liquid crystal layer 407 in an OFF state conducts, causing the second voltage 415 to be supplied to the liquid crystal pixel 405 via the second transmission gate circuit of the liquid crystal driver 404, so that the liquid crystal pixel 405 is placed in a white display mode.
The foregoing structure allows a line voltage, the first and second voltage signals, and a reference voltage to be driven by a logic voltage alone. Also, little current exept leakage current flow, because it is able to hold display screen by a data hold function of a memory circuit, in the case that the rewriting of a screen display is not necessary. Accordingly, consumption electric power can be reduced.
However, in the conventional static drive liquid crystal device, the data signals for the pair of data lines must be complementary signals having phases opposite to each other for writing data, and must be controlled to a high impedance for holding data. Thus, control of the data lines has been extremely complicated, and a circuit configuration has also been complicated.
The present invention has been made to solve the problem described above, and it is an object of the present invention to provide an electro-optic device that consumes less power, and features a simple control method and a simple control circuit configuration.
An electro-optic device in accordance with the present invention has, on a substrate, a plurality of row scanning lines and a plurality of column scanning lines that intersect with each other, a plurality of data lines provided along the column scanning lines, voltage signal lines that supplies voltage signals, and a plurality of pixel drive circuits disposed, corresponding to intersections of the row scanning lines and the column scanning lines, wherein each of the pixel drive circuits has a switching circuit that is set to a conducting mode when the row scanning lines and the column scanning lines are selected, while it is set to a nonconducting mode when at least either the row scanning lines or the column scanning lines are not selected, a memory circuit that captures data signals of the data lines when the switching circuit is in the conducting mode, while it holds data signals when the switching circuit is in the nonconducting mode, and a pixel driver that outputs a first voltage signal to the pixel from the voltage signal line when a data signal held in the memory circuit is at a first level, while it outputs a second voltage signal to the pixel from the voltage signal line when the data signal is at a second level.
The configuration in accordance with the present invention enables a line voltage, the first and second voltage signals, and a reference voltage to be driven at a level of a logic voltage. Furthermore, little current flows, because when there is no need to rewrite screen display, a display state can be held by a data holding function of the memory circuit. With this arrangement, comparison as a liquid crystal device indicates that power consumption is cmarkedly reduced as compared with the conventional passive matrix liquid crystal device. Moreover, unlike the conventional static drive liquid crystal device, it is no longer necessary to carry out the complicated control wherein data signals for a pair of data lines are set to have opposite phases for writing data, and set at a high impedance for holding data, thus providing an advantage in that a circuit configuration can be simplified.
Furthermore, the electro-optic device in accordance with the present invention may be provided with a latch circuit that captures, for each data line, data signals into associated data lines when the column scanning lines are selected, while it holds the data signals of the data lines when the column scanning lines are not selected. According to this configuration, only a selected data line produces a capacitance parasitic to an input data line, providing an advantage in that charging/discharging currents caused by changes of signals of input data lines are markedly reduced with consequent markedly reduced power consumption.
Furthermore, the foregoing electro-optic device in accordance with the present invention may consist of a pixel electrode disposed at the pixel that is a light reflective type electrode, and the pixel driving circuit may be provided under the pixel electrode via an electrical insulation film. This configuration provides an advantage in that an aperture ratio is markedly improved and a brighter easier-to-read screen can be obtained, as compared with a conventional static drive liquid crystal device in which a TFT (Thin Film Transistor) is formed on a transparent substrate, and in which an aperture ratio of a pixel has been limited by an area of a pixel driving circuit occupied in an area of one pixel.
Moreover, the foregoing electro-optic device in accordance with the present invention may be provided with a plurality of switching control circuits that output a conduction control signal to the switching circuit when the row scanning line and the column scanning line are selected, and output a nonconduction control signal to the switching circuit when at least either the row scanning line or the column scanning line are not selected, and the switching control circuits control the switching circuits in the plural pixel driving circuits. With this arrangement, a number of the switching control circuits can be reduced, and the circuit configurations and the control of the column scanning line driving circuits can be simplified. In addition, there is an advantage in that a writing operation of an entire screen can be quickly completed, permitting a reduction in power consumption.
Furthermore, the foregoing electro-optic device in accordance with the present invention is comprising a row scanning line driving circuit for supplying a row scanning signal to the row scanning line and a column scanning line driving circuit for supplying a column scanning signal to the column scanning line, and at least either the row scanning line driving circuit or the column scanning line driving circuit is constituted by a shift register circuit. This configuration provides an advantage in that a circuit configuration and control of the scanning line driving circuits can be simplified.
In addition, the foregoing electro-optic device in accordance with the present invention may be constituted by a row scanning line driving circuit for supplying row scanning signals to the row scanning lines and a column scanning line driving circuit for supplying column scanning signals to the column scanning lines, wherein at least either the row scanning line driving circuit or the column scanning line driving circuit is constitued by a decoder circuit that selects a pertinent scanning line according to an address signal of a number of bits corresponding to a number of scanning lines. With this arrangement, when only a part of display on a screen needs to be rewritten, a pixel driving circuit of only a target pixel can be controlled to rewrite a data signal, providing an advantage in that power consumption can be markedly reduced.
Furthermore, the foregoing electro-optic device in accordance with the present invention may consist of a circuit device structure in the electro-optic that device is a CMOS structure. This arrangement provides an advantage in that leakage current is no longer produced during a data holding period, making it possible to further reduce power consumption.
Moreover, an electronic equipment in accordance with the present invention may be equipped with the electro-optic device in accordance with the present invention described above. With this arrangement, an advantage is provided in which a markedly longer service life can be achieved, as compared with an electronic equipment using a conventional passive matrix liquid crystal device when performing battery drive, and a simpler control method and a simpler control circuit configuration than those in a conventional static drive liquid crystal device can be accomplished.